A novel secreted protein, NISP1, is phosphorylated by soybean Nodulation Receptor Kinase to promote nodule symbiosis

Nodulation Receptor Kinase (NORK) functions as a co-receptor of Nod factor receptors to mediate rhizobial symbiosis in legumes, but its direct phosphorylation substrates that positively mediate root nodulation remain to be fully identified. Here, we identified a GmNORK-Interacting Small Protein (GmNISP1) that functions as a phosphorylation target of GmNORK to promote soybean nodulation. GmNORK alpha directly interacted with and phosphorylated GmNISP1. Transcription of GmNISP1 was strongly induced after rhizobial infection in soybean roots and nodules. GmNISP1 encodes a peptide containing 90 amino acids with a DY consensus motif at its N-terminus. GmNISP1 protein was detected to be present in the apoplastic space. Phosphorylation of GmNISP1 by GmNORK alpha could enhance its secretion into the apoplast. Pretreatment with either purified GmNISP1 or phosphorylation-mimic GmNISP1(12D) on the roots could significantly increase nodule numbers compared with the treatment with phosphorylation-inactive GmNISP1(12A). The data suggested a model that soybean GmNORK phosphorylates GmNISP1 to promote its secretion into the apoplast, which might function as a potential peptide hormone to promote root nodulation.

Automated summary

Nodulation Receptor Kinase (NORK) acts as a co-receptor in legumes' rhizobial symbiosis, but the direct phosphorylation substrates that positively promote root nodulation have not been fully identified. This study found that GmNORK-Interacting Small Protein (GmNISP1) is a phosphorylation target of GmNORK and promotes soybean nodulation. GmNORK alpha interacts and phosphorylates GmNISP1, which is strongly induced after rhizobial infection. GmNISP1 is present in the apoplastic space and its phosphorylation by GmNORK enhances its secretion into the apoplast, potentially functioning as a peptide hormone to promote root nodulation.